Huntington's
disease (HD) is an inherited, degenerative brain disorder named after physician George
Huntington, who first described the illness in 1872. It is also
sometimes referred to as Huntington's chorea, from
the Greek word choreia
meaning dance, due to the characteristic movement
disorder that is a part of the illness. In former times it was
called St.
Vitus' Dance.

What causes HD?

HD is a genetic (hereditary) disease, meaning
that it can be passed down from one generation to the next. In
1993, scientists identified the gene that causes HD.

Genes are comprised of long strings of chemical building blocks. In
individuals with the HD gene, there are too many of these
building blocks (designated as CAG) in a particular place on chromosome 4.
The protein encoded by this gene does not work properly and eventually
leads to the symptoms of HD. Although every cell of the body has two
copies of every gene, only one abnormal or expanded gene is needed to
cause HD.

Every child of a parent who carries the HD gene has a 50:50 probability
of inheriting the abnormal gene. A child who inherits the HD gene will
eventually develop the illness, although usually not until adult life.
If a child does not inherit the HD gene, he or she will not develop the
disease, nor will his or her childen: HD does not skip
generations.

How common is HD?

About 30,000 people in North America have HD,
and another 150,000 are considered at risk for inheriting
the illness because they have (or had) a parent with HD. Most
of those affected by the illness develop it in middle adult life,
but about 10% will have onset prior to age 20 (juvenile HD) and
another 10% after age 55. Men and women are equally likely to
inherit the gene and develop the illness. HD can affect people
of all ethnic groups, but is more common among those of European
descent.